1. Field of the Invention
The present invention concerns the manufacture and measurement of active antennas comprising a large number N of parallel channels. Active antennas use this number N of channels to form the radiation diagram of the antenna by superposition of the fields resulting from the excitation of each element.
2. Description of the Prior Art
During the process of designing an antenna, theoretical calculations based on desired radio frequency characteristics determine the geometry of the radiating sources and the operating parameters of those sources and the associated active modules; these parameters include the amplifier gain, the dynamic range and/or the relative phase-shift needed to obtain the required depointing. These calculations are based on hypotheses and mathematical relations which describe physical principles and on physical data concerning the antenna and its component parts. This data must be determined or confirmed by measuring the radio frequency characteristics of the antenna.
The invention concerns a method of calibrating active antennas using near field measurements on the antenna and its radiating sources and a specific calculation to determine control parameters to be applied to the active modules and the resulting far field.
The active antenna to which the method in accordance with the invention applies may be a transmit or receive antenna or an antenna alternating between transmission and reception (e.g. a radar antenna).
In the case of a transmit antenna, the signal from a low-level centralized transmitter is divided into N supposedly identical signals on N channels by means of a power splitter. A variable gain active module on each channel then amplifies the signal and applies a variable phase-shift to the amplified signal before it is transferred to the radiating source (see FIG. 1).
In the case of a receive antenna the signal received on each radiating source is amplified and phase-shifted in a variable gain active module applying a variable phase-shift. The N amplified signals on the N channels are then combined by a power combiner and transferred over a single channel to a centralized receiver (see FIG. 2). This arrangement is the opposite of the first arrangement and, from the theoretical point of view, is strictly symmetrical to it.
In the case of antennas which alternately transmit and receive, such as radar antennas, a single device acts as the combiner for reception and as the splitter for transmission and the active modules include a device switching between a receive channel including a low-noise amplifier and a transmit channel including a power amplifier. Depending on the design of the active module, a phase-shifter and a variable attenuator are provided for each channel or if they are of the reciprocal type they may be provided on a single channel connected alternately to the two transmit/receive channels by an SPDT switch (see FIG. 3).
When designing an active antenna the control signals required for beam shaping are calculated by means of a computer using hypotheses and approximations which although they render the calculations performable do not always conform to measurable reality where the performance of the antenna is concerned.
The sources are assumed to be identical, for example, whereas in reality their radio frequency characteristics are subject to small variations due to manufacturing tolerances. The same applies to the active modules: their impedance, gain, insertion loss and phase may vary from one module to another with the result that an identical control signal does not produce exactly the same phase-shift or amplitude from one source to another.
It is also assumed that the gains are exactly the same and entirely independent of the phase and vice versa although this is not so in practise and slight influence of each on the other is inevitable.
Further, the position of a source in the array can influence the radio frequency characteristics of the source through coupling with surrounding sources. For example, the characteristics of a source at one end of the array are different than those of a more central source surrounded by neighboring sources.
Finally, the theoretical calculations assume that the source amplifiers are linear devices. This means that the resulting fields at the source can be predicted from the control signal applied to the amplifier. If the amplifier is operating close to its saturation point, which is often the case with transmission, the control signals required to obtain a given amplitude differ from those yielded by linear theoretical calculations.
The calibration method in accordance with the invention allows for these discrepancies between reality and the ideal theoretical situation in relation to far field theoretical calculations used in the characterization and design of an active antenna. The results obtained are particularly valuable for antennas having precisely shaped radiation diagrams, especially computer-driven beam shaping antennas.
The problem arising from the existence of these errors as compared with the ideal antenna made exclusively from ideal components is hardly new. The method in accordance with the invention is concerned with three types of error: spread of the radio frequency characteristics of the components (due to manufacturing tolerances), phase and gain control errors and variable coupling between radiating sources dependent on their position within the array. Prior art solutions are unsatisfactory for the reasons stated hereinafter.
To alleviate the spread of radio frequency characteristics between the modules, assumed to be identical for identical components, it is known to incorporate specific calibration circuits similar to the active antenna. In a transmit antenna these circuits sample a known fraction of the signal output by each active module and feed it to the antenna control unit. In a receive antenna these circuits inject a known signal into the receive circuit and recover it at the other end of the normal path of an antenna receive channel.
This solution has two major drawbacks: it requires a dedicated circuit for each module which significantly increases the already high price of an active antenna and the overall size, weight, electrical power consumption, heat dissipation and complexity of the system are increased accordingly. Further, the resulting calibration allows only for parameter spread affecting the circuits and neglects the effect of coupling between sources and the effect of differences between the radiating sources themselves due to manufacturing tolerances.
Another known method is to install a test antenna such as a horn or dipole antenna at a particular distance from the active antenna to be calibrated. The transfer function between the test antenna and each channel is determined by measuring the field delivered by each channel in succession using the following method. All channels except the channel under test are switched out of circuit during the measurement of the channel under test and this procedure is applied channel by channel.
Using this prior art solution requires modification to the construction of the basic active module to incorporate the function for connecting all channels except the channel under test in turn to a matched load.
One option is to maintain fixed control of the other channels while the channel in question is controlled in a variable manner, which causes the phase to rotate. In theory this enables the various phase states of the channel to be characterized.
However, this method suffers from the problem of coupling between neighboring sources, which is not measured under conditions representative of normal operation: by rotating the phase of the channel being calibrated the radiation of the other sources is disturbed slightly which disrupts the measurement of the radiated field.
The prior art has also touched on the problem of theoretical modelling of such coupling. Various models have been put forward according to the type of radiating source. The models are directed to determining by calculation the actual radiation from the source S.sub.i if surrounded by N-1 other sources S.sub.j (j.noteq.i) which are all excited by waves a.sub.j. However, the actual sources are very difficult to model correctly, especially printed circuit antennas ("patches"). However, patches are increasingly used in active antennas and the level of coupling between radiating sources of this type is particularly high.
Methods of calculating coupling theoretically are often subject to error as are methods for modifying such coupling (to reduce induced mismatching of the antenna) by coupling holes between the access guides, by the careful disposition of a dielectric radome, etc. Methods of predicting coupling theoretically should enable correction by calculation of their disturbing effects in a calibration sequence; however, in the prior art this is always independent of the measurement of parameter spread due to manufacturing tolerances or control errors.
The method in accordance with the invention can alleviate these drawbacks of the prior art and correct simultaneously the three types of error summarized above.